A static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a subject. During an MRI scan, Radio Frequency (RF) pulses generated by a transmitter coil cause perturbations to the local magnetic field, and RF signals emitted by the nuclear spins are detected by a receiver coil. These RF signals are used to construct the MRI images.
Rapidly acquiring magnetic resonance images has been successfully used to guide various modalities of radiation therapy. U.S. Pat. No. 6,198,957 describes a therapeutic apparatus which combines a linear accelerator with MRI.